The invention pertains to the field of business telephone systems with PBX capability, and, more particularly to systems that can convert conventional personal computers into machines with PBX capability
One of the problems with PBX on an expansion card technology for personal computers is lack of expandibility to add new ports above and beyond the ports provided by the expansion card. Other problems with prior systems such as the Netphone and Altigen systems (Netphone""s system is described in U.S. Pat. No. 5,659,005) are bottlenecks and latency caused by the use of a single centralized microprocessor to perform voice mail processing, outgoing message generation and other duties for all ports coupled to the card. The Netphone patent does teach time division multiplexed expansion interface 70 which can be used to couple multiple PBX cards like that shown in FIG. 2 together by a TDMA PCM a highway. This expansion capability allows multiple PBX cards to be coupled together by a TDMA bus. The main PBX card is coupled to the server by an ISA bus interface 60. An ISA bus is a slow 16-bit, master-slave type system bus used on older personal computers such as the IBM AT. An ISA bus can only transfer one word at a time between the PBX card and the host server. It is not particularly well adapted for bursting large amounts of data over a short time such as is required to playback or record voicemail recordings.
Also, any host bus requires a bus master to drive the command and address lines of the bus to control which slave device is allowed to use the bus to communicate with the master and whether the communication will be a read or write transaction. A bus slave such as the PBX card cannot control the bus and must request control of the bus such as by asserting an interrupt request line on the system bus. An interrupt request requires a context switch by the host server wherein it stores the state of all of its internal registers and program counter and vectors to an interrupt service routine which takes control of the bus and allows the PBX card to send data to the host or receive data from the host. Each such context switch causes delay.
There is no memory on the Netphone PBX card which is large enough to be capable of recording digitized voice of voicemail or IP telephony packets or other multimedia streams data such as MP3 recordings coming in from the internet via a T1 line connection. Thus, all this data that needs to be recorded must be sent to the server for recording on the host hard disk. The ISA bus will be a bottleneck in this situation and could lead to loss of data if the RAM 64 overflows.
Further, even, if multiple PBX cards are coupled together in the Netphone system, there is only one time division multiplexed bus that couples the cards together. This single TDM bus must be shared between carrying digitized voice of PBX connections between extension phones and one or more CO trunk lines or between extensions as well as outgoing or incoming voicemail data or IP telephony or multimedia data from the internet. The bandwidth requirements can exceed the available bandwidth of the bus and cause latency or lost data. Also, since voicemail recording or playback data, T1 type or IP telephony data and multimedia data is bursty traffic whereas TDMA buses are isochronous, there is a mismatch in the nature of the traffic versus the nature of the bus available to carry it. This can cause the need for additional buffer to hold the overflow thereby adding cost and delay to the process of transferring this bursty traffic from one port to another or from a port to the host server.
Another problem with current PBX systems that are built around personal computers is that in, order to provide PBX connections to a T1, DSL or ISDN digital service, an expansion card which uses up one of the precious expansion slots on the host motherboard must be used. It is disadvantageous for any PBX system to make connections to CO POTS lines or digital lines from the CO or extension lines out to extension phones (hereafter xe2x80x9cportsxe2x80x9d) through expansion cards that couple to expansion slots in the motherboard. This is because there are only a limited number of expansion slots on the host motherboard, and once they are used up, the PBX system has gotten as large as it can get and no more ports can be added without modification s to the host motherboard to add more expansion slots which may render it inoperative or slow down the system bus by loading it with parasitic capacitive and inductive loads higher than it was designed to handle for a given bit rate and clock speed.
Thus, a need has arisen for PBX system wherein a large, almost unlimited number of ports may be added in small slices by the addition of individual port expansion units to a standard, unmodified inexpensive personal computer where the port expansion units do not need to be coupled to expansion slots on the motherboard. A need has further arisen for a PBX system built around a conventional, inexpensive personal computer where the PEUs can interface either to POTS CO lines, any type of digital service to the CO or to extension lines coupled to extension phones. A need has further arisen for a PBX system built around a conventional, inexpensive personal computer where the PEUs interface to the host server through both a TDMA bus for PBX connection traffic of real time conversations and a packet switched bus carrying control and voicemail data which can easily handle the bursty and packetized nature of voicemail, ISDN, DSL and IP telephony packet-based traffic.
The genus of the invention is a class of PBX machines built around a standard, unmodified personal computer programmed with a PBX control program and to which is added one or more switch cards in expansion bus slots coupled to one or more port expansion units (PEU) which are not coupled to bus expansion slots and which couple the PBX to POTS or DSL or ISDN trunk lines or extension lines out to the various extension phones at the customer premises. Each PEU contains its own digital signal processor (DSP) so that distributed digital signal processing may be implemented to avoid any bottlenecks. One, master PEU is coupled to a switch card by a time division multiplexed (TDMA) bus as well as a packet switched control bus, and all the other PEUs, if any, are coupled to the master PEU by extensions of the TDMA and packet switched buses. The TDMA bus is used to carry port-to-port PBX real time conversations while the packet switched bus is used to carry control information, any type of packetized digital data, voicemail outbound message data packets, and inbound voicemail data packets to be recorded on the hard disk of the server. Each port expansion unit has multiple ports which can be coupled to one or more central office trunk lines or one or more lines to extension telephones. Some port expansion units (PEU) are designed to couple to T1 lines, ISDN lines or DSL lines.
The distributed DSP architecture allows a very large number of ports to be serviced and a large number of simultaneous voicemail or IP or other digital packet switched channels to be serviced without overwhelming the digital signal processing capability of the system. This is because there is a DSP in each PEU which handles processing necessary for voicemail playback and recording and any other digital signals processing necessary to support the analog or digital traffic entering and leaving its ports. Each switch card coupled to the host server motherboard also has its own DSP to handle conference calling and, in some embodiments, other processing in support of the centralized switching function. The switch cards are coupled to each other by a high capacity TDMA bus so that PBX real time conversations can be routed from one switch card to another over the TDMA bus and do not have to load down the host system bus, which is typically a PCI bus in the current generation of Pentium/Celeron class host personal computers.
The use of separate TDMA and packet switched buses to couple each PEU to the switch cards in the server allows a large number of voice mail channels to be simultaneously serviced via the packet switched bus with low or no latency without any impact on the capacity to carry PBX-connected, real time conversations on the TDMA bus coupling the PEUs to the switch cards. Further, the packet switched bus is better suited than a host bus or a TDMA bus to handle packet based bursty traffic such as is found in T1 packet based TDMA traffic, or ISDN or DSL or IP telephony packet packet based traffic as multimedia stream traffic.